The purpose of this research project is to elucidate the interrelationship that exists between fibroblast and collagen in the periodontal ligament (P.D.L.) during tooth movement. To achieve this purpose the acid and alkaline phosphatase E.M. cytochemical techniques of Barka and Anderson (1962), and Yoshiki and Kurahashi (1971-72) will be utilized to evaluate lysosomal function and collagen phagocitosis in fibroblast of the orthodontically stimulated rodent P.D.L. Maxillary right first and second molar periodontal ligaments of 30 day old Holtzman rats will be subjected to an initial orthodontic force of 20 grams for 3/4, 6 hours, 4 and 8 days. Along with their controls these ligaments will be individually incubated, stained and processed for electron microscopy. The fibroblast close to bone, middle and cemental areas of the P.D.L. will be studied in both the pressure and tension sides to determine relationships between lysosome and the so called "collagen containing profiles". The different time intervals in which the ligament cells will be studied should provide ultrastructural data on fibroblastic behavior during the initial middle and late reparative stages of tooth movement.